1. Field of the Invention
The present invention relates to a spread illuminating apparatus for use with a liquid crystal display (LCD) device, and particularly to a spread illuminating apparatus employing one or more spot-like lamp as a light source.
2. Description of the Related Art
A spread illuminating apparatus of side light type, which has a light source disposed so as to face one end surface of a light conductive plate, is heavily used as a subsidiary illuminating apparatus for an LCD device. In a side light type spread illuminating apparatus, its light source is shaped bar-like, for example a fluorescent lamp, and has a length substantially equal to the width of a light conductive plate, whereby the light conductive plate can be illuminated entirely in a uniform manner.
In a device, such as a portable telephone, which demands low power consumption, a light emitting diode (LED) is employed as a light source. The LED is a spot-like lamp and raises a problem with regard to achieving a uniform brightness spread across the light conductive plate.
A spread illuminating apparatus of side light type introduced to address the problem is disclosed, for example, in Japanese Patent Application Laid-Open No. H10-293202. In the spread illuminating apparatus disclosed therein, an end surface (light entrance surface) of a light conductive plate facing an LED is provided with a prism array structure which is adapted to laterally diffuse light emitted from the LED, and a light scattering pattern for a uniform spreading is formed on a major surface of the light conductive plate. And, a light diffuser plate may be optionally disposed between the light conductive plate and an LCD as required for achieving a further uniform illumination.
Another side light type spread illuminating apparatus to address the same problem is disclosed, for example, in Japanese Patent Application Laid-Open No. 2002-260427. In the spread illuminating apparatus, an optical element comprising an array of prisms is disposed between an LED and a light conductive plate so that light emitted from the LED is refracted differently depending on its incidence angle with respect to the prisms thereby changing the traveling direction and intensity distribution of the light when entering the light conductive plate.
Still another side light type spread illuminating apparatus is disclosed in Japanese Patent Application Laid-Open No. H10-199316 and is shown in FIG. 10 attached herein. In the spread illuminating apparatus shown in FIG. 10, an LED 1 as a spot-like light source is disposed at an end surface 2C of a light conductive plate 2, and light emitted from the LED 1 enters the light conductive plate 2 and exits from one of two major surfaces of the light conductive plate 2. Aplurality of grooves 2D extending orthogonally to the major surfaces are formed at a central portion of the end surface 2C of the light conductive plate 2 so as to oppose the LED 1, whereby light emitted from the LED 1 is diffused upon entering the light conductive plate 2.
The above-described spread illuminating apparatuses using an LED, as a light source, have the following problems.
Firstly, an optical path conversing means, which comprises, for example, arrayed prisms, or grooves as described above, must be formed at or disposed before an entrance surface of a light conductive plate so as to laterally spread light emitted from the LED. Such prisms or grooves must be arrayed at very minute interval, which requires micro-fabrication resulting in an increased cost.
Secondly, the traveling path of light emitted from the LED and once spread laterally cannot be controlled as desired, for example, cannot be collimated.
Thirdly, referring to FIG. 9 which shows a conventional spread illuminating apparatus, a light conductive plate 2 is forced to have a major surface larger than a display screen area of an LCD device (not shown) disposed over the light conductive plate 2. Specifically, what is called “a dead area 2A”, where lights emitted from LED's 1 are not uniformly spread, exists at a light entrance area of the light conductive plate 2, and has a length Ld usually ranging from 2 to 4 mm in case of a small equipment, such as a portable telephone. The dead area 2A is covered up so as to prevent light introduced in the light conductive plate 2 from exiting therefrom, thus the dead area 2A is just wasted and only an area 2B is utilized for illuminating the display screen of the LCD device (not shown), which hinders downsizing.
Lastly, referring again to FIG. 10, since an air layer exists between the LED 1 and the plurality of grooves 2D as an optical path conversing means, light emitted from the LED 1 is not fully utilized failing to provide sufficiently bright illumination. In order to compensate for resulting shortfall of brightness, power consumption must be increased thus constituting an obstacle in lowering power consumption which is strongly required.